Chapter 7 States of Matter and Changes in State
7.7CHAPTER SUMMARY AND OBJECTIVES
The state of matter (phase) in which a substance is found depends on a combination of temperature, pressure, and the strength of the intermolecular forces. Substances are gases when their thermal energy is much greater than their energy of interaction; they are liquids when their thermal energy and their energy of
interaction are comparable; and they are
solids when their energy of interaction is
much greater than their thermal energy.
The volume of a gas depends upon its pressure, its temperature, and on the number of
moles present. The pressure of a gas can be determined by the height of a mercury column that it will support. As a result, gas pressures are often given in mm Hg or torr. At sea level, the pressure is 760 torr, which is the same as 1 atmosphere. The relationship between the pressure, volume, temperature, and number of moles of gas is PV = nRT, the ideal gas law. Mixtures of gases are homogeneous, so they are solutions. The molar concentration of a component of a solution is defined as the number of moles of the component per liter of solution. The molar concentration of A is defined as [A] = n/V. A
Each component of a mixture of gases exerts
its own pressure, called the partial pressure,
which is directly proportional
to its molar concentration (P
=RT[A]). Dalton’s law of A
partial pressures states that the total pressure in a mixture of gases is the sum of the partial pressures of the component gases. The average th
ermal energy of a system of molecules is
approximated as RT, which is about 2.5 kJ/mol
at room temperature. Thermal or kinetic
energy is a measure of the how fast the mo
lecules are moving, rotating, and vibrating.
Intermolecular forces are much weaker than
covalent bonds and are responsible for
holding the molecules in the liquid and solid
states. They are of three types: dispersion
forces, present in all compounds; dipole-dipole forces
, found only in polar molecules; and
hydrogen bonds
, which dramatically change the properties of molecules with N-H, O-H
and F-H bonds.
The potential energy of a molecule increase
s (becomes less negative) in going from
solid to liquid to gas. Thus, the phase of a
substance can be changed by the addition of
energy: a solid can be melted to a liquid or
sublimed to a gas, and a liquid can be
vaporized to a gas by the addition of heat.
A substance at its melting point undergoes a
dynamic equilibrium between solid and li
quid phases, which is represented as solid
U
liquid. The liquid
U
vapor equilibrium is also a dynamic equilibrium. The pressure of the
vapor in equilibrium with a liquid is referre
d to as the vapor pressure. The boiling point of
a liquid is the temperature at which the vapor
pressure equals the external pressure and is,
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